Pulmonary and Extrapulmonary Effects of Increased Colloid Osmotic Pressure During Endotoxemia in Rats

Colloid osmotic pressure (COP) is generated across membranes that are permeable to water and low-molecular-weight substances but that are impermeable to large molecular compounds, such as plasma proteins. Fluid administration is a fundamental part of resuscitation therapy, which is performed by using either crystalloid or colloid solutions. Crystalloid solutions, such as Ringer’s lactate solutions and saline solutions, supply water and sodium to intravascular and extravascular compartments. Colloid solutions, such as those containing albumin, dextrans, or starches, increase the plasma COP and may shift fluid from the interstitial compartment to the intravascular compartment. Increased capillary permeability promotes the distribution of water from the intravascular to the interstitial space, leading to the development of multiple-organ edema, including pulmonary edema. This occurs in a wide variety of clinical disorders, including ARDS, sepsis, and septic shock, as well as head or body trauma. Animal models demonstrate that a multitude of endogenous vasodilator mediators, including oxygen free radicals, prostaglandins, leukotrienes, and tumor necrosis factor, are associated with increased capillary permeability. A continuing controversial question for the treatment of critically ill patients is which of the two major strategies, namely, colloid or crystalloid therapy, should be used to ameliorate the effects of increased capillary permeability and hypovolemia. generic claritin

Fluid transport across capillaries is a very complex chemophysical process, taking place at variable stages of different critical-care conditions. In general, fluid transport is defined by the Starling equation as a function of the following variables: (1) microvascular (capillary) hydrostatic pressure; (2) perimicrovascular hydrostatic pressure; (3) plasma COP; (4) COP of the interstitial fluid; (5) fluid conductance across capillary membrane (coefficient of filtration); and (6) the reflection coefficient (measures the extent to which the semipermeable membrane prevents egress of plasma proteins). A further dynamic factor, which is not included in the Starling equation, is the ability of the lymphatic system to clear fluid from the interstitium. Alterations in these forces influence the interstitial fluid accumulation (ie, edema formation),